SEAWATER COMPOSITION

Water in the world’s oceans and seas contains on average 3.5 per cent dissolved salts. This defines the salinity, usually expressed in parts per thousand as 35‰. brackish water has a salinity less than 25‰, and hypersaline water has a salinity greater than about 40‰. The origin of the solutes in seawater is a combination of terrigenous products introduced by rivers and outgassing from the Earth’s interior by volcanism. Chloride ions and sodium ions make up more than 85 per cent of the dissolved substances in seawater (see Table). Other major constituents, some essential for life in seawater, include sulfate, magnesium, calcium and potassium (K): there are also minor quantities of bicarbonate, bromide, borate, strontium and fluoride. Although the concentration of these ions in seawater may vary, their relative proportions remain virtually constant: they form the conservative constituents of seawater. The proportions of other, non-conservative constituents are tied to biological, seasonal or other short-term earth cycles and vary in space and time. nutrients include nitrogen, phosphorus (P) and silicon (Si): these are essential for biogenic activity, and their concentrations are measured in parts per million. The quantities of gases dissolved in seawater are strongly affected by biogenic activity, particularly photosynthesis. Concentrations of trace elements are measured in parts per billion. They may aid or retard life in the oceans, and human activity is responsible for most of their levels in shallow-water depths.

[See alsoevaporite, hydrothermal vent]